An III-V group nitride compound semiconductor typified by gallium nitride (GaN), so-called a nitride semiconductor, has been attracting attention. The nitride semiconductor is expressed by the following general expression: InxGayAl1−x−yN (0≦x≦1, 0≦y≦1, x+y≦1), and is a compound semiconductor comprising: at least one of indium (In), gallium (Ga), and aluminum (Al) included in group-III elements; and nitrogen (N) included in group-V elements. In the field of devices using such a nitride semiconductor, especially light-emitting devices which convert electricity into light are developed actively.
There are roughly two kinds of semiconductor light-emitting devices. One is a light emitting diode (LED) which converts injected carriers (electrons, holes) into light through spontaneous emission. The other is a semiconductor laser, such as a laser diode (LD), in which a waveguide is provided in the device and carriers injected in the waveguide are converted into light through stimulated emission.
A light-emitting diode using the nitride semiconductor has been developed actively as a white LED combined with phosphor for a backlight light-source of a lighting apparatus and a liquid crystal display apparatus. In contrast, regarding a semiconductor laser using the nitride semiconductor, a blue-violet laser diode which emits laser beam having an emission wavelength of 400 nm to 410 nm is used as a light-source of a record and reproduction apparatus for blu-ray discs.
Furthermore, in recent years, a nitride semiconductor light-emitting device in which emission wavelength is elongated from a blue region to a green region has also been developed and manufactured as a light-source for a display. For this purpose, a nitride semiconductor light-emitting device which emits light capable of reducing speckle noise that is a disadvantage of laser beam, such as a super luminescent diode (SLD), has also been developed.
The nitride semiconductor light-emitting device such as the semiconductor laser or the SLD realizes a high efficiency light-emitting device, by generating stimulated emission light in the optical waveguide as described above. To obtain the stimulated emission light efficiently, it is required to increase a light confinement coefficient of the optical waveguide. As a scheme for increasing the light confinement coefficient, in the conventional technique for example, a scheme can be raised in which GaN is used as a guide layer and AlxGa1−xN (0<x≦1) is used as a cladding layer. In this case, it is possible to increase the light confinement coefficient by increasing Al composition in AlxGa1−xN in the cladding layer which significantly reduces the refractive index than that of GaN in the guide layer, thereby increasing a difference in the refractive indexes.
However, increasing the Al composition in the cladding layer entails side effects. Particularly, in a p-type cladding layer provided above a light emitting layer, ionization energy of an Mg acceptor used as dopant increases in proportion to the Al composition. Therefore, there is a problem in that it becomes difficult to realize a high hole-concentration in the p-type cladding layer, which causes an increase in a series resistance in the cladding layer and an increase in the operating voltage of the nitride semiconductor light-emitting device.
In order to solve the above problem, Patent Literature (PTL) 1 discloses a laser diode which is said to be able to realize high light confinement while realizing low operating voltage. The following describes the structure of the conventional laser diode disclosed in PTL 1, with reference to FIG. 17.
As shown in FIG. 17, a conventional laser diode 2100 includes: a substrate 2101 comprising, e.g., sapphire substrate; and an n-type contact layer 2110, an n-type lower cladding layer 2130, an n-type lower waveguide layer 2140, a multiple quantum well (MQW) region 2150, a p-type confinement layer 2160, and a p-type upper waveguide layer 2170, formed above the substrate 2101.
Furthermore, an upper cladding layer 2180 comprising a transparent conductive film is formed on the upper waveguide layer 2170 and located over an active region 2155 of the MQW region 2150. Moreover, a pair of isolation layer portions 2185 is formed on opposite sides of the upper cladding layer 2180.
Furthermore, a p-side electrode 2190 comprising metal is formed on the upper cladding layer 2180 and the isolation layer portions 2185. In contrast, an n-side electrode 2120 comprising metal is formed on a first exposed region of the n-type contact layer 2110.